Preheat oven to 275°F. Beat egg whites with an electric mixer on medium speed until foamy, about 1 minute; beat in cream of tartar. Increase speed to high, and beat until medium peaks form, about 2 minutes. Add 1 cup of the granulated sugar, 1 tablespoon at a time, beating well after each addition. Beat on high speed until stiff peaks form, about 2 to 3 minutes.

Spread meringue on bottom and up sides of a 9-inch glass or ceramic pie plate coated with cooking spray. Sprinkle chopped walnuts over edge of meringue, lightly pressing to adhere. Bake in preheated oven until lightly browned, about 1 hour. Cool completely on a wire rack, about 30 minutes.

Whisk together egg yolks, lemon juice, salt, 2 1⁄2 tablespoons of the zest, and remaining 1⁄2 cup granulated sugar in the top of a double boiler. Cook over simmering water, stirring constantly, until mixture thickens, about 8 to 10 minutes. Place top of double boiler in a bowl of ice water, and let stand, stirring occasionally, until completely chilled, about 20 minutes. Transfer mixture to a medium bowl.

Beat cream and powdered sugar with electric mixer on high speed until medium peaks form, about 3 to 4 minutes.

Stir 1⁄2 cup of the whipped cream into egg-citrus mixture. Gently fold 1 cup of the whipped cream into egg-citrus mixture. Gently fold 1 cup of the mandarin orange segments into mixture. Transfer to prepared meringue pie shell, smoothing top. Mound remaining whipped cream on top of pie, making a well in the center. Arrange remaining orange segments on top center of pie. Sprinkle with remaining 1⁄2 tablespoon orange zest. Chill 2 to 3 hours before serving.